Apparatus for determining distance

ABSTRACT

An electronic apparatus for depth sounding and range finding and primarily intended for use with ships. The apparatus produces an analogue voltage indicative of distance and proportional to the time duration between the transmission of pulses of acoustic energy and the reception of corresponding reflection pulses and includes circuitry for producing warning signals if the analogue voltage reaches a pre-set value indicative of a certain distance or if the rate of change of the analogue voltage is predictable as producing a future value indicative of a certain distance within a pre-set time interval. Further circuitry is provided to improve the validity of the operational results of the apparatus.

0 United States Patent [1 1 [11] 3,766,518 Rilett Oct. 16, 1973APPARATUS FOR DETERMINING 2,403,527 7/1946 Hershberger 34011 x DISTANCE2,998,591 8/1961 Lovett 3,179,939 4/1965 Castellini 343/112 [75]Inventor: John W. Rilett, Saffron Walden,

England Primary Examiner-Richard A. Farley [73] Assignee: Maurice PowellLimited, Walden, Anomeysllghl'ue, Rothwell, MiOn, Zilm nd EnglandMacpeak 22 F1 (1: D .15 1969 l 1 cc [57 ABSTRACT Appl. No.: 884,893

Foreign Application Priority Data Dec. 18, 1968 Great Britain 60,196/68U.S. Cl. 340/3 R, 343/7 TA Int. Cl. G0ls 9/68 Field of Search 340/1, 3;343/7 A,

343/7 TA, 112 CA, 13

References Cited UNITED STATES PATENTS 7/1935 Turner 340/3 10/1965 Olsonet al. 343/7 TA PULSE INTERVAL I/ZO GENERATOR 21 DEPTH RANW (SetsTRANSMITTER An electronic apparatus for depth sounding and range findingand primarily intended for use with ships. The apparatus produces ananalogue voltage indicative of distance and proportional to the timeduration between the transmission of pulses of acoustic energy and thereception of corresponding reflection pulses and includes circuitry forproducing warning signals if the analogue voltage reaches a pre-setvalue indicative of a certain distance or if the rate of change of theanalogue voltage is predictable as producing a future value indicativeof a certain distance within a pre-set time interval. Further circuitryis provided to improve the validity of the operational results of theapparatus.

14 Claims, 6 Drawing Figures (Transmitted pulse) sequence "keel echoes)(Selected echo) 23 9 DEPTH l) (Blanks Tx pulse and 9 power) (Echosignal) 5 f (All echoes;

Sea 'bottoml/ Logic levels) 40 DEPTH RAN E RANGE (Sets scale Zi- DEPTHANALOGUE factor) GENERATOR 5O (gets iength (Converts echo time of 'v') w(viii) to voltage depth) KEE CORRECTION (Finds correct echo and routesit) ,(Depth AnalOgJe voltage) (xii) ADAPTIVE 9 CIRCU'TS (Sets operatingconditions) (Set prediction 36 time) DEPTH METER DEVICE COMPARATORPREDIOTOR MEMORY DEPTH THRESHOLDQ POTENTIOMETER 38 To warni (Set memorytime) (Signal if below set time) ng or control circuits DEVlCE (Signalif below set depth) Depth increasing SHEETSUFS FIG. 5.

PATENTEDUEI i 6 1975 d (depth) depth warning time required (xvii) Towarning or control circuits TRieER DEVICE User control (50) (xvi) FIG.6.

TIME

Time of warning (xii) X (xv) (xvii) 5 Depth analogue voltage APPARATUSFOR DETERMINING DISTANCE BACKGROUND OF THE INVENTION The presentinvention relates to a method of, and an apparatus, for measuring thedistance from one point to an object by means of wave propagation.

A common method of measuring distance is to transmit a short pulse ofenergy, which may be acoustic (i.e. a pressure wave) or electromagneticso that the propagated wave will travel to the object, the distance ofwhich is to be measured. The wave will be reflected by the object andthe reflected wave is then received. By knowing the velocity of the wavein the medium concerned, and by measuring the time taken for the wave totravel to the object and back, the distance of the object may becalculated. Examples using this known technique are radar, radioaltimeters, laser rangeflnders, and sonar systems.

An apparatus utilizing this known method usually aims to merely give theuser an indication of the distance from the object which can be asurface. In certain application such as with aircraft, ships or landvehicles it is desirable to give a user further information moreparticularly a warning when the distance decreases below a set level,and/or a warning when the rate of decrease of distance might result incollision within a preselected time period.

A general object of this invention is to provide a method and apparatuscapable of providing this extra information.

SUMMARY OF THE INVENTION According to one aspect of the invention thereis provided a method of determining distance, comprising transmittingpulses of energy, receiving said pulses when reflected by an object,ascertaining the time duration between the transmission of said pulsesand the reception of the corresponding reflection, producing anelectrical signal indicative of distance and producing a warning signalwhen said electrical signal reaches a pre-selected value.

Further according to the invention a prediction signal is produced whenthe rate of change of said electrical signal is indicative of apre-determined distance being reached within a pre-selected timeinterval.

According to another aspect of the invention there is provided means fortransmitting pulses of energy, means for receiving said pulses whenreflected from an object, means for continuously ascertaining the timeduration between the transmission of said pulses and the reception ofthe corresponding reflections to thereby produce an electrical signalindicative of distance and means for producing a warning signal whensaid electrical signal reaches a pre-determined value.

The electrical signal is preferably an analogue voltage and preferablythere is further provided means for producing a prediction signal whenthe rate of change of said analogue voltage is indicative of apredetermined distance being reached within a preselected time interval.

The invention also aims to improve the validity of thedistance-measuring technique in two ways: firstly, by preventing atemporary loss of the reflected pulses from resulting in erraticoperation, and secondly, by reducing the susceptibility of the apparatusto spurious signals caused by interference or objects in the echo path.

The invention is particularly, but not solely, directed to a sonardepth-indicating system for use in small ships and power boats. In thisapplication a conventional ultrasonic link is used, both the transmitterand receiver being mounted on the underwater portion of the ship orboat, and directing the transmitted pulses either slightly forwards anddownwards or directly downwards from the ship or boat.

A normal transmission and reception system is used in which short burstsof acoustic energy are transmitted into the water at regular timeintervals, these intervals being set by means of a depth range control.

The invention may be understood more readily, and various other featuresof the invention may become more apparent, from consideration of thefollowing description.

BRIEF DESCRIPTION OF DRAWINGS A constructional embodiment of anapparatus for use in depth-sounding with boats will now be described, byway of example only, and with reference to the accompanying drawings,wherein:

FIG. 1 is a perspective view of the outer parts of the apparatus as seenby a user;

FIG. 2 is a block schematic diagram of the overall circuitry andcomponents of the apparatus;

FIG. 3 is a block schematic diagram showing certain parts of thecircuitry and components of the apparatus in greater detail;

FIG. 4 is a graphical voltage/time representation of various waveforms(i) to (xi) arranged with identical time abscissae;

FIG. 5 is a further graphical voltage/time representation of variouswaveforms (xii) to (xvii) arranged with identical time abscissae; and

FIG. 6 is a block schematic diagram of the predictor and memory deviceof FIG. 2.

DESCRIPTION OF PREFERRED EMBODIMENT As shown in FIG. 1 the externalfeatures of the apparatus consist of a main housing or case 1 preferablyrotatably carried on a base member 2 with an electrical power input lead3 and a cable 4 connectible to a transmitter/receiver transducer head '5which is intended for mounting on the hull of a boat. The facia of thecase I is provided with a window 6 which can be selectively illuminatedto give a visual warning simultaneously with an audible warning producedby a loudspeaker 13 disposed within the case 1. This warning is givenwhen a depth, pre-selected by a user, is reached and to this end amanually-operable setting member 7 provided with a calibrated scale ofdepths is provided. A manually operable volume control member 10 is usedto vary the volume of the audible signal produced by the loudspeakerwhich signal may compose periodic bleeps.

A main indicating instrument 8 in the form of a meter gives a visibleindication of actual depth and its sensitivity, i.e. range, iscontrolled by a row of manuallydepressible keys 9. A further row ofmanuallydepressible keys 11 with a corresponding row of panels 12 isused to pre-set various time intervals as a basis for a further warningfunction. This warning function is concerned with a prediction in termsof time as to when a particular depth sounding, usually of zero depth,can be expected. When the pre-set time of prediction, set by depressingone of the keys 11, is reached thecorresponding panels in the row ofpanels 12 is illuminated and a different audible tone, which again canconsist of bleeps," is delivered by the loudspeaker 13.

The transducer head 5 is preferably so mounted so that its direction maybe varied, say through 90. Thus the apparatus may be used to sound indepth, or to sound in range: the latter may be used for example in lowvisibility and if appropriate with a scale-switching facility tocompensate for the dissimilar speeds of sound or other wave motion inwater and in air, to range another vessel, a pierhead, or exposed landor rocks. By setting the transducer head 5 at a selected angle, a vectorsounding may be read, which again may be useful in some circumstancesfor example if approaching a submerged vertical or steep ledge orrapidly-shoaling coastline.

The operation of the apparatus will become more apparent as theapparatus itself is described in more detail and in conjunction withFIGS. 2 to 6 of the accompanying drawings.

Referring to FIGS. 2 to 5 and initially to FIG. 2 a pulse generator 20generates synchronized pulses, shown as waveform (i) in FIG. 4, therepetition rate of which pulses is determined by the selected keysetting of the row of keys 9. This repetition rate will vary from about200 pulses per second at depths less than l feet to about 2 pulses persecond at depths in the order of 1,000 feet. These pulses, waveform (i)energize a transmitter 21 which produces pulses in the form of bursts ofacoustic energy, shown as waveform (ii) in FIG. 4, at the pulserepetition rate of the waveform (i). The power of these acoustic pulsesdepends upon the setting on the range-selecting member 9. The acousticpulses, waveform (ii) are delivered to the transducer head andtransmitted away from the boats hull and generally downwards towards thesea-bed S. The transducer head 5 also serves to receive attenuatedacoustic energy reflected from the sea-bed and any intervening objects,and the reflections are fed to a receiver 22 employing amplifying andfiltering circuits and an age. system all known per se. The waveform(iii) in FIG. 4 shows a typical echo signal which can be received by thetransducer head 5. This typical echo signal, waveform (iii), which isfed to a receiver 22 is depicted as being composed of transmitterbreakthrough X a spurious unwanted echo Y and a steady echo Z from theseabed and representing the desired information. The receiver 22 issuitably controlled to eliminate the breakthrough X as will be describedhereinafter and produces an output which is fed to a block referred toas adaptive circuits 40 the details of which are shown in FIG. 3. Theoutput from the generator 20, waveform (i), is also fed to a blockreferred to as a depth analogue generator 23 the details of which areshown in FIG. 3.

As shown in FIG. 3 the depth analogue generator 23 includes a keelcorrection monostable device 24 which receives the waveform (i) and isenergized by each of the pulses. The output of the monostable device 24is shown as waveform (v) in FIG. 4 and is used to bias or delay thetime-measuring circuitry, by means of a preset control 50 set by a user,to thereby enable the meter 8 to indicate the depth of the sea-bed froma point at some distance below the transducer head 5. Thus, for example,a yachtsman would normally have the transducer head 5 fitted to the hullof his yacht but could set the monostable device 24 by means of thecontrol 50 so that the instrument 8 would actually indicate the depthbetween the bottom of the keel of his yacht and the sea-bed. In additionto this function, the output from the monostable device 24, waveform (v)is fed back to the receiver 22 to inhibit the receiver from operationfor the duration of each pulse produced thereby and thus serves to blankout the transmitter breakthrough X. The remaining constituents Y and Z,waveform (iii), of the input to the receiver 22 are filtered therein andproduce squared pulses of corresponding duration at the output of thereceiver 22. The output waveform of the receiver 22 is shown as waveform(iv) in FIG. 4. The pulse D in the waveform (iv) corresponds to thedesired echo signal Z whereas the pulse U in the waveform (iv)corresponds to the unwanted spurious signal Y which is eliminated by amasking pulse the generation of which is described hereinafter.

The trailing edge of each pulse from the device 24 is used to trigger abi-stable device 25 into its on" state, say its 1 state. The leadingedge of the pulse D in the waveform (iv) normally serves to re-set thedevice 25 to its of state, i.e. in this case its 0 state and is subjectto a gating operation described hereinafter. The output of the device 25is shown as waveform (vi) in FIG. 4.

The output from the generator 20, waveform (i), is also fed tovoltage-controlled monostable device 26 which produces an output shownas waveform (ix) in FIG. 4 referred to as the masking pulse. Theduration between the lagging edge of each masking pulse, waveform (ix)and the lagging edge of the corresponding output pulse of the bi-stabledevice 25, waveform (vi), is used to produce an inspection pulsedepicted as waveform (x) in FIG. 4. To this end the outputs from thedevices 25, 26 are fed to a gating device 27. The output from the device27, waveform (x), is fed to a ramp generator 28 which produces an outputdepicted as waveform (xi) in FIG. 4 and referred to as the inspectionramp. The voltage to which the ramp portion of the inspection ramp,waveform (xi), rises is directly proportional to the duration of theinspection pulse, waveform (x).

The output from the generator 28 is fed to a sample and hold circuit 29which is adapted to store the output voltage of the generator 28 untilcaused to convey this voltage by the application of a sample pulsedepicted as waveform (viii) in FIG. 4. The generation of the samplepulse will be described hereinafter.

The output from the circuit 29 is fed through a lowpass filter 30 to thedevice 26 and serves to ensure that an increase in the duration of theinspection pulse, waveform (x), produces a corresponding increase in theduration of the masking pulse, waveform (ix).

The output from the device 26, waveform (ix), is also fed to a furthergating device 31 the output of which is connected to the re-set input ofthe bistable device 25. The gating device 31 serves to allow thewaveform (iv) to re-set the device 25 except during the duration of themasking pulse, waveform (ix), when false echoes could inadvertentlyre-set the device 25.

A control voltage taken from the output of the filter 30 is applied tothe voltage-controlled-monostable device 26 and may be made proportionalto the time interval during which the bistable device 25 is in the 1state, and therefore could be used as a voltage analogue of depth.However, it may be more convenient to generate the voltage analogue ofdepth separately, as will now be described.

The output from the device 25 wave (vi), is also fed to a delay circuit32 a further gating device 34 and a further ramp generator 33. The rampgenerator 33 is set by the output from the monostable device 24 andproduces an output voltage, depicted as waveform (vii) in FIG. 4,proportional to the duration of the pulse produced at the output of thedevice 25, waveform (vi). The gating device 34 also receives the outputfrom the device 24, waveform (v), and produces an output depicted inFIG. 4 as waveform (viii) and referred to as the sample pulse. Theoutput from the ramp generator 33, waveform (vii), is fed to a furthersample and hold circuit 35 which is adapted to store the maximum valueof the output voltage of the generator 33 and which is controlled by thesample pulse produced at the output of the gating device 34, waveform(viii).

The gating device 34 ensures that the duration of the sample pulse,waveform (viii) is equal to the duration between the lagging edge ofcorresponding output pulse from the device 25, waveform (vi) and theleading edge of the next pulse from the device 24, waveform (v). For theduration of the sample pulse waveform (viii) both circuits 29, 35 arerendered operative to deliver the voltage stored thereby. The outputfrom the circuit 35 is fed through a filter and buffer circuit 36 toprovide part of a depth-indicative analogue voltage depicted in FIG. 5as waveform (xii).

If the desired echo occurs much earlier than it occurred in a previouscycle it will be blocked by the masking pulse, waveform (ix), and thebistable device 25 will not be reset and thus no sampling pulse,waveform (viii) will be delivered to the delay circuit 32. If thisoccurs for more than a few cycles of operation, the delay circuit 32will act effectively to short out the control voltage, from the filter30, to the voltage-controlled-monostable device 26. This will cause theduration of the masking pulse to be reduced until the echo signal againpasses through the gating device 31.

Occasionally, an echo may not be received at all due to a poorreflecting surface, or an interruption in the path of the acoustic beam.In this situation, the bistable device 25 will not be reset, and thesample-and-hold circuits 29, 35 will not transfer their respectivevoltages. In this event the voltage-controlled-monostable device 26 willcontinue to produce a masking pulse of the same duration as was producedbefore the echo disappeared and until the delay circuit 32 again comesinto operation.

Should an echo not be received, no sample pulse will be produced (asexplained previously), and so the sample part of the circuit 35 will notoperate. As a result, the hold part of the circuit 35 will continue tostore a voltage analogue of the depth measured when the last echo wasreceived.

The waveform (xii) is fed to three items, namely:

The instrument or meter 8, a comparator device 36 (FIG. 2) and apredictor and memory device 37 (FIG. 2).

The comparator device 36 is used to energize the visual and audibledepth warnings referred to hereinbefore. The depth analogue voltage,waveform (xii) in FIG. 5 is compared continuously in the comparatordevice 36 with a voltage, depicted in FIG. 5 as waveform (xiii), set bythe depth threshold potentiometer 38 (FIG. 2) controlled by the settingmember 7 shown in FIG. I. The reference a in waveforms (xii) and (xiii)denote equality. The voltage applied to the potentiometer 38 is madeequal to the depth analogue voltage required to drive the meter 8 tofull-scale deflection and the voltage taken from the potentiometer 38 tothe comparator device 36 is proportional to the depth set in by thesetting member 7, known as the warning depth. When the depth analoguevoltage becomes less than the voltage from the potentiometer 38, thecomparator device 36 produces an output shown as waveform (xiv) in FIG.5 which can be used to trigger the audible and visual warnings. As afurther possibility the output could also be used to affect the drivingcontrols of the boat, e.g. by reducing the engine throttle setting.

The predictor and memory device 37 provides a means of computing thefunction: (Actual Depth/Time of Warning Required) Rate of change ofdepth with time If the value of this function becomes negative, anoutput is to be produced, which may be used to operate theaforementioned warning signal at the loudspeaker l3 and one panel in therow of panels 12. Under certain conditions it may be desirable tocontinuously compute the time to collision, and to display this on asuitable indicating device. However, in this embodiment of theinvention, only the former alternative is employed as shown in FIG. 6.

As shown in FIG. 6, the depth analogue voltage, waveform (xii) isattenuated by an attenuator 40 to obtain the first term of the abovefunction, which is shown as waveform (xv) in FIG. 5. The degree ofattenuation is determined by the time of warning selected by the user onthe keys 11. The attenuated voltage, waveform (xv), is then added in anadding circuit 41 to a voltage which is a measure of the rate of changeof the depth analogue voltage waveform (xii) with time and which isshown in FIG. 5 as waveform (xvi). The references b in waveforms (xv)and (xvi) denote equality. The differentiation function is accomplishedby using a feed-.

back amplifier 42 in the manner known per se. In order that thepredictor device 37 may measure the overall slope of the sea bottom andnot make premature predictions based on small sharp changes of slope, itis necessary to pass the summed voltage waveforms (xv) (xvi) through alow-pass filter 43 to smooth out small fluctuations in voltage. Theextent of the filtering is adjusted in two ways; firstly increasing thetime of warning required is arranged to automatically increase the timeconstant of the filter 43 and secondly the user may control the timeconstant by means of an external control member 50.

After filtering the combined signal is fed to a trigger circuit 44 whichgives an output when its input becomes negative. This signal, shown aswaveform (xvii) may then be used to energize one of the panels in therow of panels 12 and possibly to trigger other warning or controlcircuits.

I claim:

1. In an apparatus for determining distance, said apparatus comprisingmeans for transmitting successive pulses of energy, means for receivingsaid pulses when reflected from an object, means for continuouslyascertaining the time duration between the transmission of said pulsesand the reception of the corresponding reflections to produce anelectrical analogue signal indicative of the distance between theapparatus and said object, and means for producing a warning signal whensaid electrical analogue signal reaches a predetermined value; theimprovement comprising means for producing a prediction signalindicative of a predetermined distance being reached within apredetermined time interval by continuously assessing the rate of changeof said electrical analogue signal.

2. An apparatus according to claim 1, wherein there is further providedmeans for varying the repetition rate of said transmitted pulses inaccordance with preselectable ranges of distance and an instrument forindicating said electrical signal.

3. An apparatus according to claim 2, wherein a monostable device isprovided to cause the indicating instrument to indicate the distancebetween the object and a point displaced from a transducer head servingto transmit and receive said pulses and to selectively inhibit theoperation of said receiving means during the transmission of pulses, andwherein the monostable device produces a square-wave pulse of pre-setduration upon transmission of each of said pulses.

4. An apparatus according to claim 3, wherein the receiving meansproduces square-wave pulses indicative of echo reflection pulses and themeans for ascertaining the said time duration at least includes abi-stable device adapted to produce an output pulse triggered by thelagging edge of each pulse produced by said monostable device and re-setby the leading edge of a succeeding echo reflecting pulse produced bythe receiving means and corresponding to said pulse produced by themonostable device.

5. An apparatus according to claim 3 and further comprising means forproducing masking pulses each of which prevents indicative operation ofthe apparatus during its duration.

6. An apparatus according to claim 1 and further comprising means forproducing masking pulses each of which prevents indicative operation ofthe apparatus during its duration.

7. An apparatus according to claim 5, wherein there is provided meansfor producing an inspection pulse the duration of which is equal to theduration between the lagging edge of one of said masking pulses and thelagging edge of the corresponding output pulse from said bi-stabledevice and wherein there is also provided means for determining theduration of each inspection pulse.

8. An apparatus according to claim 7, wherein the means for determiningthe duration of each inspection pulse is a ramp voltage generator, and afeed back loop extends between the ramp generator and said maskingpulseproducing means, the feed back loop serving to apply a control voltageto said masking-pulse producing means.

9. An apparatus according to claim 8, wherein the ramp generator isconnected to a sample and hold circuit adapted to store the maximumvalue of the output voltage of said ramp generator and there is providedmeans for producing a sample pulse the duration of which is equal to theduration between the lagging edge of the corresponding output pulse ofsaid bi-stable device and the leading edge of the succeeding pulse fromsaid monostable device, the sample pulse being produced only when thebistable device is reset by an echo reflection pulse and being conveyedto the sample and hold circuit to cause the latter to deliver thevoltage stored thereby.

10. An apparatus according to claim 9, wherein the output from thesample and hold circuit produces said control voltage and a delaycircuit is connected between said control voltage input of themasking-pulse producing means and the output of the bistable device,said delay circuit being selectively operable to short circuit thecontrol voltage if several echo reflection pulses become blocked by saidmasking pulses.

1 1. An apparatus according to claim 1, wherein there is provided apotentiometer for setting a voltage corresponding to the value of saidanalogue voltage which would be indicative of a certain distance, andmeans for producing a warning signal includes a comparator adapted toindicate equivalence between the analogue voltage and the voltage set bysaid potentiometer.

12. An apparatus according to claim 1, wherein the means for producing aprediction signal is in the form of a computing device for continuouslycomputing an estimated time before collision with the object from therate of change of said electrical analogue signal and means forcomparing the output from said device with a manually preselected timeinterval representing the time of warning desired.

13. An apparatus according to claim 12, wherein said computing deviceincludes a differentiating circuit.

14. in an apparatus for determining distance, said apparatus comprisingmeans for transmitting successive pulses of energy, means for receivingsaid pulses when reflected from an object, means for continuouslyascertaining the time duration between the transmission of said pulsesand the reception of the corresponding reflections to produce anelectrical analogue signal indicative of the distance between theapparatus and said object, and means for producing a warning signal whensaid electrical analogue signal reaches a predetermined value; theimprovement comprising means for producing a prediction signal bycontinuously assessing the rate of change of said electrical analoguesignal, said prediction signal being indicative of a predetermineddistance being reached within a predetermined time interval, and meansfor eliminating spurious echo reflections and for correcting fortemporary loss of reflection pulses.

1. In an apparatus for determining distance, said apparatus comprisingmeans for transmitting successive pulses of energy, means for receivingsaid pulses when reflected from an object, means for continuouslyascertaining the time duration between the transmission of said pulsesand the reception of the corresponding reflections to produce anelectrical analogue signal indicative of the distance between theapparatus and said object, and means for producing a warning signal whensaid electrical analogue signal reaches a predetermined value; theimprovement comprising means for producing a prediction signalindicative of a predetermined distance being reached within apredetermined time interval by continuously assessing the rate of changeof said electrical analogue signal.
 2. An apparatus according to claim1, wherein there is further provided means for varying the repetitionrate of said transmitted pulses in accordance with pre-selectable rangesof distance and an instrument for indicating said electrical signal. 3.An apparatus according to claim 2, wherein a monostable device isprovided to cause the indicating instrument to indicate the distancebetween the object and a point displaced from a transducer head servingto transmit and receive said pulses and to selectively inhibit theoperation of said receiving means during the transmission of pulses, andwherein the monostable device produces a square-wave pulse of pre-setduration upon transmission of each of said pulses.
 4. An apparatusaccording to claim 3, wherein the receiving means produces square-wavepulses indicative of echo reflection pulses and the means forascertaining the said time duration at least includes a bi-stable deviceadapted to produce an output pulse triggered by the lagging edge of eachpulse produced by said monostable device and re-set by the leading edgeof a succeeding echo reflecting pulse produced by the receiving meansand corresponding to said pulse produced by the monostable device.
 5. Anapparatus according to claim 3 and further comprising means forproducing masking pulses each of which prevents indicative operation ofthe apparatus during its duration.
 6. An apparatus according to claim 1and further comprising means for producing masking pulses each of whichprevents indicative operation of the apparatus during its duration. 7.An apparatus according to claim 5, wherein there is provided means forproducing an inspection pulse the duration of which is equal to theduration between the lagging edge of one of said masking pulses and thelagging edge of the corresponding output pulse from said bi-stabledevice and wherein there is also providEd means for determining theduration of each inspection pulse.
 8. An apparatus according to claim 7,wherein the means for determining the duration of each inspection pulseis a ramp voltage generator, and a feed back loop extends between theramp generator and said masking-pulse producing means, the feed backloop serving to apply a control voltage to said masking-pulse producingmeans.
 9. An apparatus according to claim 8, wherein the ramp generatoris connected to a sample and hold circuit adapted to store the maximumvalue of the output voltage of said ramp generator and there is providedmeans for producing a sample pulse the duration of which is equal to theduration between the lagging edge of the corresponding output pulse ofsaid bi-stable device and the leading edge of the succeeding pulse fromsaid monostable device, the sample pulse being produced only when thebistable device is reset by an echo reflection pulse and being conveyedto the sample and hold circuit to cause the latter to deliver thevoltage stored thereby.
 10. An apparatus according to claim 9, whereinthe output from the sample and hold circuit produces said controlvoltage and a delay circuit is connected between said control voltageinput of the masking-pulse producing means and the output of thebistable device, said delay circuit being selectively operable to shortcircuit the control voltage if several echo reflection pulses becomeblocked by said masking pulses.
 11. An apparatus according to claim 1,wherein there is provided a potentiometer for setting a voltagecorresponding to the value of said analogue voltage which would beindicative of a certain distance, and means for producing a warningsignal includes a comparator adapted to indicate equivalence between theanalogue voltage and the voltage set by said potentiometer.
 12. Anapparatus according to claim 1, wherein the means for producing aprediction signal is in the form of a computing device for continuouslycomputing an estimated time before collision with the object from therate of change of said electrical analogue signal and means forcomparing the output from said device with a manually preselected timeinterval representing the time of warning desired.
 13. An apparatusaccording to claim 12, wherein said computing device includes adifferentiating circuit.
 14. In an apparatus for determining distance,said apparatus comprising means for transmitting successive pulses ofenergy, means for receiving said pulses when reflected from an object,means for continuously ascertaining the time duration between thetransmission of said pulses and the reception of the correspondingreflections to produce an electrical analogue signal indicative of thedistance between the apparatus and said object, and means for producinga warning signal when said electrical analogue signal reaches apre-determined value; the improvement comprising means for producing aprediction signal by continuously assessing the rate of change of saidelectrical analogue signal, said prediction signal being indicative of apredetermined distance being reached within a predetermined timeinterval, and means for eliminating spurious echo reflections and forcorrecting for temporary loss of reflection pulses.